Gas flow meters are used for measuring the flow or quantity of a moving gas.  The basis of good flowmeter selection is a clear understanding of the requirements of the particular application. Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit and taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid's velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate.

The basic relationship for determining the liquid's flow rate in such cases is:

Q= V x A; where Q = liquid flow through the pipe V = average velocity of the flow A = cross-sectional area of the pipe.

Other factors that affect liquid flow rate include the liquid's viscosity and density. Specific applications should be discussed with a flow meter manufacturer before purchasing to ensure proper fit, form and function. Flow sensors include a wide variety of flow controllers, switches, meters and detectors. They are designed to analyze flow quality, meter flow rates, or detect leaks of both liquids and gases. Gas sensors and analyzers are used to measure the amount of a specific gas or gases in a given environment, or, as with gas chromatography, are used to separate them. Aside from sensors, instruments and switches, residual gas analyzers and total organic carbon analyzers are included in this family.